Increasing demand for gasoline and diesel requires more petroleum products of light and middle range distillates which can be mixed into gasoline and diesel pools. However, currently available hydrocarbon resources, most commonly include crude oil and other heavy fractions and heavy fraction distillates, requiring refining processes to generate desired products.
Conventional refining processes upgrade heavy oil into light and middle distillate range products with the aid of thermal energy, catalysts, and hydrogen. Representative conventional processes include catalytic hydroprocesses and coking processes. Catalytic hydroprocessing, such as hydrocracking, produces clean gasoline and diesel products, where impurities, such as sulfur, are minimized, but the premium quality of the products requires a huge consumption of hydrogen to produce. Coking processes, where catalysts and hydrogen are not employed, utilize thermal cracking reactions to upgrade heavy oil into gases, light distillates, and middle distillates, but also produce large amounts of low economic byproducts, such as solid coke.
A third option to upgrade heavy oil is the use of supercritical water. A low dielectric constant makes supercritical water a good solvent for organic compounds. Supercritical water has been used as a reaction medium for certain chemical reactions such as oxidation and for upgrading hydrocarbons. Supercritical water is a good reaction medium for upgrading because hydrogen can be transferred from the water to the hydrocarbons. Thus, a huge supply of hydrogen gas is not necessary. Supercritical water acts as a diluent, diluting the hydrocarbons. In upgrading heavy oil using supercritical water, as in thermal cracking, a radical is generated due to chemical bonds breaking. Molecular rearrangement follows radical propagation, including cracking, dimerization, and oligomerization. However, unlike in thermal cracking alone, upgrading reactions in supercritical water reduce the chance for radicals to be oligomerized, because the supercritical water acts as a “cage” to restrict the radicals. Radical species are stabilized by supercritical water through the cage effect (i.e., a condition whereby one or more water molecules surrounds the radical species, which then prevents the radical species from interacting). Stabilization of radical species is believed to help to prevent inter-radical condensation and thus, reduce the overall coke production in the current invention. For example, coke production can be the result of the inter-radical condensation.
Coke, or petroleum coke, is a solid material formed in upgrading reactions. The solid material may leave the reactor with the liquid products, but commonly remains as a layer on the inner surfaces of the reactor and process piping. To be useful, coke requires further processing and is therefore considered a less valuable by-product to upgraded hydrocarbons.
Coking is a significant problem in upgrading reactions. Coking increases at increased temperatures. While the extent to which coking will occur is hard to predict, it is known that temperatures above 400° C. are enough to form coke. One reason coking is accelerated at higher temperatures is because radical formation is accelerated at higher temperatures. More radicals results in more oligomerization reactions, which increases the molecular condensation reactions or coke formation.
Hot spots contribute to coking in upgrading reactors. Hot spots occur due to localized heating of a metal surface, such as a reactor wall. In general, localized heating is caused by an irregular or non-uniform distribution of a direct heat source, such as a flame, an electric heater, a formation of an insulator on a metal surface, or an irregular fluid distribution on a metal surface. An example of an irregular fluid distribution on a metal surface would be the stoppage of process fluid flow in a tubular reactor. Thus, a furnace or heater must be designed for uniform distribution of temperature through the reactor walls. One design feature is to coat the surfaces of reactors with heat transfer materials to provide better heat distribution, but such heat transfer materials often have short life spans and are expensive to replace. A second design option is to ensure a high superficial velocity of the process fluid through the reactor. A high superficial velocity can improve temperature distribution. However, in some cases designing for high superficial velocity requires a high length to diameter ratio of the reactor tube which increases the cost of the reactor due to the material weight of the reactor tube. Any design should feature sensitive instrumentation for temperature monitoring throughout the reactor to prevent formation of hot spots. However, even with precise design and instrumentation, hot spots in a direct heating system are inevitable. At best, a reactor design can hope to minimize the number and intensity of hot spots.
Hot spots contribute to coking because they cause localized excessive heating of the fluid in the reactor. The excessive heating causes localized coke formation on the inner wall of the reactor. Once the coke forms on the inner wall both the size and intensity of the hot spot can increase leading to more coke formation. Additionally, the coke formation hinders accurate measurement of temperature in the reactor.
Coke formation during upgrading processes limits the functionality of the upgrading process. A reduction in the coke formed during upgrading would lead to an increased yield of liquid hydrocarbon products. Coke formation limits the run length, or residence time, the petroleum can spend in the reactor. Shorter residence times results in less efficient upgrading. Coke plugs the process lines causing an increase in the pressure of the process lines and the reactor. If the pressure increases above a certain point, the entire process must be shut down so the coke can be removed. Otherwise the pressure build-up could cause mechanical failure of the plant equipment. Coke formation is one of the common causes for unscheduled shut-downs of refining processes.
Supercritical water reduces coke formation compared to a purely thermal process. The extent of coking prevention by supercritical water, however, depends on the type of heavy oil. Even supercritical water is limited in preventing coke formation because molecules, especially, heavy molecules, are not easily dissolved in supercritical water due to their low solubility, so the larger molecules, such as asphaltenes, can be easily converted to coke through radical mediated reactions. Additionally, supercritical water at higher temperatures has lower density than heavy oil and that density changes more quickly as the temperature rises at supercritical pressures. At 25 MPa, the density of water at 400° C. is 166.54 kg/m3 while at 450° C. the density is 108.98 kg/m3. The relative difference in the density of heavy oil and supercritical water causes settling of heavy molecules on the reactor bottom or walls, where such segregated heavy molecules act as a precursor for coke formation. Even in supercritical water reactor, the aggregation of heavy molecules can lead to the formation of coke, and coke can lead to the formation of hot spots.
A supercritical water process that reduces or prevents the formation of hot spots would be advantageous. A supercritical water process that reduces the formation of coke and increases operational stability over conventional supercritical water processes would be advantageous.